Sand castings using calcium oxide compounds

ABSTRACT

A method for cleaning residual mold material from a sand casting, the mold material comprising sand, a binder, and an electrolytic material, the method comprising exposing the residual mold material to an electrolytic solution; and subjecting the residual mold material to an electric current.

BACKGROUND OF THE INVENTION

The methods and apparatus disclosed herein relate to the manufacture of metal objects using sand casting processes.

DESCRIPTION OF RELATED ART

Sand casting is a metal casting process in which sand is used to form a mold that receives and shapes molten metal. The molten metal cools and solidifies within the mold, to form a rough casting of the metal object being desirably fabricated. The metal object produced by the sand casing process is commonly referred to as a “sand casting.” Sand castings are usually manufactured in specialized factories called foundries. Metals commonly used in sand casting include, for example, iron, steel, bronze, brass, aluminum, magnesium alloys, and various alloys that include lead, tin, and zinc.

The molds used to manufacture sand castings are formed from a mixture of sand and a bonding agent. The bonding agent is combined with the sand to give the resulting aggregate mixture sufficient strength and plasticity to be used as a mold. Types of sands commonly used to form sand molds include, for example, silica, olivine, chromite, zircon, and chamotte. Typical binder materials include, for example, clay and water, oil, resins, sodium, and silicate. The specific types of sand and binder material used in a particular application depend on the metal or metal alloy being cast.

A typical sand mold has one or more cavities that receive and shape the molten metal. The sand mold is formed in a frame or box-like structure referred to as a flask. A typical flask has a bottom portion and a top portion, known respectively as the “drag” and the “cope.” Cavities are created in the mold by compacting or otherwise packing the aggregate (consisting of sand and binding material) around a pre-formed pattern placed in the flask. The pattern has a shape corresponding to the exterior shape of the metal object to be manufactured. The pattern is generally slightly larger than the metal object to be fabricated to account for contraction of the molten metal during cooling. Alternatively, the cavity may be formed by carving the desired shape of the metal object to be fabricated directly in the aggregate sand-binder material.

A sand mold is usually formed by filling the drag with the sand-binder aggregate. The pattern is placed in the aggregate, and the cope then is mated, namely placed in facing contact with the drag. Additional aggregate material is added to fill the assembled flask, and the aggregate is compacted around the pattern. The cope is subsequently removed, the pattern is withdrawn from the compacted aggregate, and then the cope is replaced so that the sand “face” of the drag facingly contacts a correspondingly sand “face” of the cope, leaving a cavity that defines the exterior of the object to be formed by casting.

The mold may include one or more additional components known as cores. Cores typically are formed from the same sand-binder aggregate used to form the main portion of the mold. Cores are used to produce hollow features within the casting. For example, cores may be used to define internal passages or channels that carry cooling fluid within engine blocks and cylinder heads that have been formed by sand casting iron or steel. The cores are placed in the bottom half of the mold once the pattern has been removed. The cope is then mated with the drag to complete the formation of the mold.

Molten metal is introduced into the mold cavity via passages formed in the mold for this purpose. The metal is allowed to cool and solidify into a rough casting having external and internal geometries defined by the mold. The flask is then removed, and the sand is broken up and cleared from the exterior of the rough casting. The sand-binder material that formed the cores is broken up by rods or shots, and removed from the casting. Metal risers that may have been used to support the casing are then cut from the casting. The rough casting may be subjected to heat treatment and other processes that relieve stresses induced by the cooling of the molten metal and increase the hardness of the casting.

It is common for the sand-binder aggregate material to become fused to the surfaces of the rough casting as a result of the heat and pressure associated with the casting process. This residual aggregate material can be difficult to remove. Removing residual aggregate material from the interior passages and cavities of the casting can be especially challenging. For example, it is often difficult to access the entire length of passages or cavities during the cleaning process; and it can be difficult to remove the fused material without scratching or otherwise damaging the adjacent surfaces of the casting. This should be evident from FIG. 1, which is a diagrammatic cross-sectional view of the open sand “face” of one-half of a mold for sand casting of a casing for a hydraulic pump. As shown in FIG. 1, the casing has a number of interior passages. These passages, and especially those with curved or otherwise non-linear geometry, tend to trap and thereby inhibit removal of the mold material.

SUMMARY OF THE INVENTION

Methods are provided for producing and cleaning sand castings formed from ferrous and nonferrous metals and metal alloys. An electrolytic chemical, such as sodium bicarbonate, is included in the aggregate of sand and binder material used to form the mold. Sands and binder materials, in general, are not electrically conductive. Including an electrolytic chemical such as sodium bicarbonate makes the sand-binder aggregate electrically conductive. When the now electrically-conductive aggregate is subjected to an electric current, the current facilitates dissolution of the aggregate material. Thus, aggregate material that has been fused to the surfaces of the metal casting can be dissolved or otherwise loosened, and then flushed away.

Inventive sand molds formed from a mixture of sand, binder material, and an electrolytic material are also an aspect of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic cross-sectional view of the sand “face” side of one-half of mold prepared for sand casting a casing for a hydraulic pump.

FIG. 2 is a flow chart depicting an exemplary process for forming and cleaning a sand casting in accordance with the invention.

DETAILED DESCRIPTION

The attached figures are not drawn to scale; they are provided to illustrate the invention. Several aspects of the invention are described below. Well-known structures and operations are not shown in detail to avoid obscuring the invention. The invention is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement the invention.

FIG. 2 is a block diagram of the steps involved in an exemplary method for forming and cleaning sand castings made of ferrous and nonferrous metals and metal alloys. An electrolytic chemical, such as sodium bicarbonate, is included in the aggregate of sand and binder material used to form the mold used in the sand casting process. Molten metal is poured into the mold, and upon cooling and solidifying, forms a rough casting. Upon solidification and cooling, the aggregate material in the mold is subjected to an electric current. The electric current is conducted through the aggregate mold material, which is electrically conductive due to the presence of the electrolytic material therein. The electric current facilitates dissolution of the aggregate material, which in turn releases the material from the surfaces of the rough casting.

The electrolytic material can be present in the sand-binder aggregate used to form the main portion of the mold, as well as the mold cores. The techniques disclosed herein thus can be used to facilitate removal of residual aggregate mold material from the exterior and interior surfaces of the casting.

The electrolytic chemical, such as sodium bicarbonate, can be added to the sand before the sand is mixed with the binder. Alternatively, or in addition, the electrolytic chemical can be added to the binder before the binder is mixed with the sand. In applications where the sand and binders are provided in a premixed state, the electrolytic chemical can be added to the premix. The sand can be any type of coarse or fine sand that is suitable for use with the specific type of ferrous or non-ferrous metals being cast. These types of sands may include, for example, silica, olivine, chromite, zircon, and chamotte. The sand may be smooth-grained or sharp-grained sand, or a mix thereof. The binder likewise can be any binder that is suitable for use with the type of alloy being cast. Suitable binders may include, for example, clay and water; oil; resins; sodium; or silicate.

The use of sodium bicarbonate as the electrolytic material is preferred in some applications due to its relatively low melting point, which facilitates melting of the sodium bicarbonate as the molten metal is introduced into the mold. The melting of the sodium bicarbonate allows it to more fully combine with the sand-binder aggregate, which in turn can increase the electrical conductivity of the aggregate. The use of sodium bicarbonate is disclosed for exemplary purposes only; other types of electrolytic chemicals can be used in the alternative.

The electrolytic solution or alkaline bath can be formed by dissolving potassium carbonate in water, so that the aqueous potassium carbonate solution has a pH of approximately 12. The use of aqueous potassium carbonate solution with a pH of 12 is disclosed for exemplary purposes only; other types of electrolytic solutions, and other pH values, can be used in the alternative.

The mixture of sand, binder, and sodium bicarbonate can be formed into a mold in the manner described above in relation to the conventional sand molding process. The mixture of sand, binder, and sodium bicarbonate is used to form both the main portion of the mold, and any cores being used to define internal passages or cavities within the casting under production. Once assembly of the mold has been completed, the molten metal can be poured into mold in the above-described manner.

The cleaning process can commence after the molten metal has cooled and solidified to form the casting, the rough casting has been removed for the mold, and most of the aggregate mold material has been broken away or otherwise removed from the exterior of the casting. During the cleaning process, the casting, and the attached residual mold material, can be immersed in a bath of the electrolytic solution. Alternatively, the electrolytic solution can be sprayed onto the casting and the residual mold material.

Electric current in the form of direct current can be applied to the residual material in the presence of the electrolyte. The casting can be electrically connected to the anode of a source of direct current. The cathode of the direct-current source can be immersed in the electrolytic aggregate. (Alternatively, the casting can be connected to the cathode of the current source, and the cathode can be immersed in the electrolytic aggregate.) Because the residual mold material is electrically conductive due to the presence of the sodium bicarbonate therein, the current source, casting, electrolytic solution, and residual material form an electric circuit. The resulting flow of electric current through the residual mold material promotes dissolution of the material, so that the material fused to the exterior and interior surfaces of the casting is released. The dissolved or otherwise loosened mold material is flushed away in a subsequent rinsing step. A source of alternating current can be used in lieu of the direct-current source noted above.

As noted above, residual core mold material fused to interior surfaces within of the casting can be particularly difficult to remove using conventional methods. The removal of residual core material from interior surfaces, using the techniques disclosed herein, can be further enhanced by positioning a conductive rod within each core as the core is formed. The rod can be electrically connected the positive or negative terminal of the direct current source, so that the interior region of the core is directly exposed to electric current. This arrangement enhances the flow of current through the core, which in turn can result in even more effective removal of the residual core material from the internal surfaces.

The techniques disclosed herein can be used, for example, to remove residual aggregate mold material from a hydraulic pump casting measuring 10″×10″×3″, with six internal cavities and three internal channels, where the channels have a 180° turn with ¼-inch radius. The residual sand and binder materials that typically fuse to the interior surfaces that form these features can be difficult to remove, and if not removed can render the casting unusable.

Although the use of sodium bicarbonate as the electrolytic material may be satisfactory in many applications, its use may present disadvantages in certain applications. The relatively low melting point of the sodium bicarbonate may cause it to vaporize upon contacting the molten material being poured into the mold, which can create defects in the mold or roughen the casting. Also, any sand reclaimed from the cleaning process may include residual sodium from the sodium bicarbonate. This is significant because the sand used in sand casting processes is often reclaimed and sold for use in making concrete, and the presence of sodium can render the sand unsuitable for forming concrete.

In a preferred embodiment of the above-described method of the invention, powdered or finely ground calcium oxide (CaO) is added to the sand and binder material in lieu of or in addition to sodium bicarbonate. The calcium oxide may be obtained from limestone that is preferably about 99% (ninety-nine percent) or greater pure calcium carbonate.

The calcium oxide may make up by weight or by volume between about ½% (one-half percent) and about 5% (five percent) of the aggregate mixture, the balance being sand and binder material. The sand may be smooth grained or sharp grained sand, or a mix thereof.

The melting temperature of calcium oxide is relatively high, and exceeds that of the ferrous and non-ferrous materials commonly used to form sand castings, including aluminum, brass, bronze, iron, copper, gold, lead, magnesium, nickel, silver, steel, tungsten, zinc, and the like. The calcium oxide thus can remain in solid form throughout the casting process, and does not undergo a phase transformation to a vapor or liquid phase that otherwise could generate defects in the mold or the resulting casting. Also, the presence of calcium oxide in the sand reclaimed from the electrolyte does not render the reclaimed sand unsuitable for use in making concrete.

After casting, the rough casting is immersed in an alkaline bath or electrolytic solution, and electrical current is passed through the casting as described above. The electrolytic solution can be aqueous potassium carbonate solution with a pH of approximately 12. Other types of electrolytic solutions, and other pH values, can be used in the alternative.

In another alternative to the above method, powdered or finely ground limestone (calcium carbonate) can be used in lieu of calcium oxide. Calcium carbonate has a melting point that is lower than that of calcium oxide, but higher than that of sodium bicarbonate. The calcium carbonate may make up by weight or by volume between about ½% (one-half percent) and about 5% (five percent) of the mixture, the balance being sand and binder material.

In other alternative methods, other sodium salts, such as sodium chloride, can be used in lieu of sodium bicarbonate, calcium oxide, or calcium carbonate in forming the aggregate of sand, binder, and electrolytic material. In yet other possible embodiments, two or more of sodium salts, calcium oxide, and calcium carbonate can be used together as the electrolytic material. 

What is claimed is:
 1. A method for cleaning residual mold material from a sand casting, the mold material comprising sand, a binder, and an electrolytic material, the method comprising exposing the residual mold material to an electrolytic solution; and subjecting the residual mold material to an electric current.
 2. The method of claim 1, wherein the electrolytic material comprises sodium bicarbonate.
 3. The method of claim 1, wherein the electrolytic material comprises calcium oxide.
 4. The method of claim 1, wherein the electrolytic material comprises calcium carbonate.
 5. The method of claim 1, wherein the electrolytic material comprises sodium chloride.
 6. The method of claim 1, wherein the electrolytic material comprises a sodium salt.
 7. The method of claim 1, wherein the electrolytic material comprises two or more of: a sodium salt; calcium oxide, and calcium carbonate
 8. The method of claim 1, wherein the electrolytic solution comprises aqueous potassium carbonate.
 9. The method of claim 1, wherein the electrolytic solution has a pH of approximately
 12. 10. The method of claim 1, wherein exposing the residual mold material to an electrolytic solution comprises immersing the casting and the residual mold material in a bath of the electrolytic solution.
 11. The method of claim 1, wherein exposing the residual mold material to an electrolytic solution comprises spraying the residual mold material with the electrolytic solution.
 12. The method of claim 1, further comprising rinsing the sand casting after subjecting the residual mold material to the electric current.
 13. The method of claim 1, wherein subjecting the residual mold material to an electric current comprises electrically connecting the sand casting and the electrolytic solution to a source of direct current.
 14. The method of claim 13, wherein electrically connecting the electrolytic solution to a source of direct current comprises placing a cathode or an anode of the source of direct current in the electrolytic solution.
 15. The method of claim 1, wherein subjecting the aggregate to an electric current comprises electrically connecting the electrolytic solution, and an electrically-conductive member within a core of the mold, to a source of direct current.
 16. A method for creating a sand casting, comprising: forming a cavity in an aggregate material comprising sand, a binder, and an electrolytic material, the cavity being shaped as an approximate negative impression of an exterior shape to be formed in the sand casting, forming a core from the aggregate material, the core having a shape approximating a desired shape of an internal passage or cavity to be formed in the sand casting; pouring a molten metal into the cavity, where the molten metal, upon cooling and solidification, forms the sand casting; exposing residual aggregate material on the sand casting to an electrolytic solution; and subjecting the residual aggregate material to an electric current.
 17. The method of claim 16, wherein the electrolytic material comprises sodium bicarbonate.
 18. The method of claim 16, wherein the electrolytic material comprises calcium oxide.
 19. The method of claim 16 wherein the electrolytic material comprises calcium carbonate.
 20. The method of claim 16, wherein the electrolytic material comprises sodium chloride.
 21. The method of claim 16, wherein the electrolytic material comprises a sodium salt.
 22. The method of claim 16, wherein the electrolytic material comprises two or more of: a sodium salt; calcium oxide, and calcium carbonate.
 23. The method of claim 16, wherein the electrolytic solution comprises aqueous potassium carbonate.
 24. The method of claim 16, wherein the electrolytic solution has a pH of approximately
 12. 25. The method of claim 1, wherein exposing residual aggregate material on the sand casting to an electrolytic solution comprises immersing the casting and the residual aggregate material in a bath of the electrolytic solution.
 26. The method of claim 16, wherein exposing residual aggregate material on the sand casting to an electrolytic solution comprises spraying the residual aggregate material with the electrolytic solution.
 27. The method of claim 16, further comprising rinsing the sand casting after subjecting the residual aggregate material to the electric current.
 28. The method of claim 16, wherein subjecting the residual aggregate material to an electric current comprises electrically connecting the sand casting and the electrolytic solution to a source of direct current.
 29. The method of claim 28, wherein electrically connecting the electrolytic solution to a source of direct current comprises placing a cathode or an anode of the source of direct current in the electrolytic solution.
 30. The method of claim 16, wherein: forming a core from the aggregate material comprises forming the core around an electrically-conductive member; and subjecting the residual aggregate material to an electric current comprises electrically connecting the electrically-conductive member to a source of electric current.
 31. A sand mold, comprising a mixture of sand; a binder; and an electrolytic material.
 32. The sand mold of claim 31, wherein the electrolytic material comprises sodium bicarbonate.
 33. The sand mold of claim 31, wherein the electrolytic material comprises calcium oxide.
 34. The sand mold of claim 33, wherein the electrolytic material comprises calcium carbonate.
 35. The sand mold of claim 31, wherein the electrolytic material comprises sodium chloride.
 36. The sand mold of claim 31, wherein the electrolytic material comprises sodium salt.
 37. The sand mold of claim 31, wherein the electrolytic material comprises two or more of: a sodium salt; calcium oxide, and calcium carbonate. 